1. Field of the Invention
The present invention relates to a thin film magnetic head comprising a magneto-resistive effect element for reading as a signal a magnetic field strength of a magnetic recording medium or the like, and further relates to a head gimbal assembly and a hard disk drive each including such a thin film magnetic head.
2. Description of the Related Art
In recent years, following improvement in areal recording density of a hard disk drive, improvement in performance of a thin film magnetic head has been required. As the thin film magnetic head, use has been widely made of a composite thin film magnetic head having a structure wherein a reproducing head comprising a read-only magneto-resistive effect element (hereinafter may also be referred to simply as “MR element”), and a recording head comprising a write-only induction-type electromagnetic transducer element are stacked relative to a substrate.
As the MR element, there can be cited an AMR element using an anisotropic magneto-resistive effect, a GMR element using a giant magneto-resistive effect, a TMR element using a tunnel-type magneto-resistive effect, or the like.
As the GMR element, a spin-valve GMR element has been often used. The spin-valve GMR element comprises a nonmagnetic layer, a soft magnetic layer formed on one surface of the nonmagnetic layer, a ferromagnetic layer formed on the other surface of the nonmagnetic layer, and a pinning layer (generally an antiferromagnetic layer) formed on the ferromagnetic layer on its side apart from the nonmagnetic layer. The soft magnetic layer is a layer that acts to change its magnetization direction depending on a signal magnetic field from the external. The ferromagnetic layer is a layer of which a magnetization direction is fixed by a magnetic field from the pinning layer (antiferromagnetic layer).
Large output and small Barkhausen noise are required as characteristics of the reproducing head. Generally, for reducing the Barkhausen noise, a bias magnetic field is applied to the MR element in a longitudinal direction (hereinafter, this bias magnetic field will be referred to as “longitudinal bias magnetic field”). The application of the longitudinal bias magnetic field to the MR element is carried out by, for example, disposing bias magnetic field applying layers each in the form of a permanent magnet, a stacked body of a ferromagnetic layer and an antiferromagnetic layer, or the like on both sides of the MR element.
On the other hand, in order to cope with further increase in magnetic recording density, there have been required reduction in track width of the MR reproducing head and reduction in shield gap length representing a distance between two shield layers disposed on upper and lower sides of the MR element.
As described above, in the MR reproducing head, the bias magnetic field applying layers such as the permanent magnets are disposed on both sides of the MR element to cancel magnetic domains of the magnetism sensitive layers (particularly the soft magnetic layer) by a longitudinal bias magnetic field applied from the bias magnetic field applying layers, thereby to suppress the Barkhausen noise.
However, because of the gap narrowing and track narrowing of the MR reproducing head, it has been getting difficult to effectively apply the longitudinal bias magnetic field to the magnetism sensitive layers (particularly the soft magnetic layer). Particularly, there has arisen a problem that the Barkhausen noise becomes more liable to occur as the track width is reduced.
In order to solve such problems, JP-A-H10-312512 and JP-A-H10-312514 have proposed that FeCo magnetic underlayers are interposed at connecting portions between bias magnetic field applying layers each made of a Co hard magnetic layer and an MR element. It is described that, by providing the FeCo magnetic underlayers, magnetic separation between the bias magnetic field applying layers and the MR element can be avoided so that a stable and effective bias magnetic field can be applied to magnetism sensitive layers.
However, as a result of assiduous studies by the present inventors, it has been known that, in the foregoing proposed structure, presumably based on the fact that c-axis in-plane orientation of the Co hard magnetic layers each formed on the FeCo underlayer is degraded, a coercive force Hc in an in-plane direction (direction parallel to a film surface) of each bias magnetic field applying layer made of the Co hard magnetic layer tends to be reduced. Therefore, even in case of the foregoing proposed structure, if further gap narrowing or track narrowing is aimed, there arises a tendency that the effective bias magnetic field can not be applied, thus resulting in possibility that occurrence of the Barkhausen noise can not be effectively suppressed.
The present invention has been made under these circumstances and has an object to provide a thin film magnetic head that can maintain at a high level a coercive force Hc in an in-plane direction of each of bias magnetic field applying layers so that even when further gap narrowing or track narrowing is aimed, an effective bias magnetic field can be applied to thereby suppress occurrence of the Barkhausen noise, and has a further object to provide a head gimbal assembly and a hard disk drive each comprising such an improved thin film magnetic head.